The present invention generally relates to photolithography, and more specifically relates to a method for optimizing wafer edge patterning.
The present invention deals with the need to maintain wafer flatness at the edge of the wafer so as to accurately print images in focus at the edge of a wafer so that edge die will yield. In particular, the present invention deals with the need to accomplish this while using masks with small-field patterns. In some schemes, this is key to optimizing yield.
Existing approaches to solving this problem include attempting to prevent edge topography by tightly controlling CMP performance at the wafer edge. Unfortunately, there are limits on how much performance can be extracted from a CMP process. Furthermore, optimizing CMP wafer edge topography sufficiently to improve edge die imaging fidelity may well have adverse impacts on other CMP parameters, such as within-die uniformity.
Another approach involves exposing edge images with focus tilts so as to try to pre-compensate for wafer edge topography, thereby reducing defocus effects. However, attempting to pre-compensate for edge tilts is speculative at best. Wafer edge topography tends to be random in nature, both in terms of its occurrence, its location, and its severity. Any tilts introduced may improve some edge dies, but will very likely make others worse.
Yet another approach involves exposing the current image in “dummy shots” adjacent to, but exterior to, the edge dies. Dummy shots are typically “partial shots,” with all dies at least partly outside the resist EBR ring or even off the wafer, and therefore incapable of yielding. However, by exposing dummy shots, CMP processes see a more uniform pattern further out on the wafer, reducing the impact of CMP-induced wafer edge topography (which may be caused in part by a transition from patterned wafer surface to unpatterned wafer surface) on the actual yieldable edge dies. Subsequent layers will therefore see less wafer edge topography.
Exposing dummy shots tends to be fairly effective at improving edge die patterning on successive layers. On large-field masks, dummy shots can be placed adjacent to virtually all edge dies. On small field masks, however, limitations on the travel range of the exposure tool's stage mean that many edge dummy shots which might be desired simply cannot be exposed, because the proper image on the mask cannot be placed over top of the particular location on the wafer.